This invention relates generally to the detection of ground faults in ungrounded power systems, and more specifically concerns the use of zero sequence current from both ends of the line to achieve sufficient sensitivity so that ground faults can be detected in spite of the very small phase-ground fault current in ungrounded systems.
Most power systems in the United States are grounded systems, either solidly or by a low, impedance connection. Grounded power systems are used to minimize voltage and thermal stresses on the power system, provide for personal safety, reduce communication system interference and promote rapid detection and elimination of ground faults because of large fault currents, which can be quickly identified. Grounded power systems do reduce overvoltage stresses on the system; the large fault current magnitudes, however, are a severe disadvantage. Phase-to-ground faults must be cleared immediately to avoid thermal stress on the system, wire-based communication channel interference and safety hazards for any individuals in the vicinity of the phase-to-ground fault. Accordingly, power service must be interrupted, in the event of phase-to-ground faults, even though the fault may be temporary, i.e. transient.
An alternative to grounded power systems is, of course, ungrounded systems, which are used in many foreign countries and many large industrial plants in the U.S. Ungrounded systems restrict the ground fault current and achieve most of the above power system goals, with the exception of the minimization of voltage stress. The disadvantage of an ungrounded system is that phase-to-ground faults produce only relatively small ground fault currents which are hard to detect and therefore create a sensitivity problem for those relays arranged to detect grounded system faults.
In ungrounded systems, the neutral has no intentional connection to ground and the system is connected to ground through the line-to-ground capacitances. Single line-to-ground faults shift the neutral system voltage, but leave the phase-to-phase voltage triangle intact. Accordingly, if all the loads are connected phase-to-phase, the loads do not suffer from a reduced voltage and can continue operation during single phase-to-ground faults, at least until a fault occurs on another phase.
For ungrounded systems, the major factors that limit the magnitude of ground fault current, which is normally used to detect ground faults, are the zero sequence line-to-ground impedance and the fault resistance. Zero sequence or three single phase voltage relays can detect ground faults in ungrounded systems; however, such an approach is not very popular because it is not selective, i.e. all the relays across a power system will measure virtually the same zero sequence voltage for a single phase-to-ground fault. With such systems, locating and isolating the fault requires sequential disconnection of the feeders in turn, and then determining that the zero sequence voltage has returned to its pre-fault value in order to identify the fault.
A sensitive directional ground varmetric element is a typical alternative to sequential disconnection of feeders. In these systems, zero sequence voltage and current are measured at the local relay location. A forward fault declaration from the directional elements, combined with a communications-assisted scheme of tripping logic, to create an assisted tripping scheme, produces trip decisions at relatively high speed when the relays at both ends determine the fault as being in the forward direction. However, when one relay on the line does not make a directional declaration due to insufficient current, the operating speed of the system slows significantly, which is a disadvantage in the ground fault detection system.
Accordingly, it would be desirable, using zero sequence impedance, to determine phase-to-ground faults both quickly and with high sensitivity.
Accordingly, the invention is a ground fault detection system for use in a local protective relay for ungrounded power systems, comprising: a selected one of (a) a zero sequence voltage value from a local relay on a protected power line and (b) a zero sequence voltage from a remote relay on the power line; total zero sequence current values from the local relay and from the remote relay; a circuit for calculating zero sequence impedance from the selected zero sequence voltage value and the total zero sequence current value; and a first tripping circuit for tripping a circuit breaker associated with the protected power line at a selected time interval following determination that the calculated zero impedance value exceeds a selected threshold, the first tripping circuit being subject to a second tripping circuit for tripping the circuit breaker at a time interval greater than the selected time interval when the calculated zero impedance exceeds a preselected value and when selected other circuit conditions exist.